9 Nystagmus and Ocular Oscillations in Infancy and Childhood Richard W. Hertle ye care practitioners may be among the first to evaluate Einfants and children with involuntary ocular movements. Pediatric ophthalmologists may, in fact, see more patients with nystagmus than any other specialist because of the frequent association of nystagmus with strabismus.9,16,20,42,54,61,75 Nystag- mus may be covered less frequently in literature and research because there is less we understand or can do about it, compared to strabismus or other childhood eye diseases. HISTORICAL PERSPECTIVE Nystagmus is a rhythmic, involuntary oscillation of one or both eyes. The term comes from the Greek word “nystagmos,” to nod, drowsiness and from “nystazein,” to doze; probably akin to Lithuanian “snusti,” also to doze. Using the information obtained from a complete history, physical examination, and radiographic and oculographic evaluations, more than 40 types of nystagmus can be distinguished (Table 9-1). Some forms of nystagmus are physiological whereas others are pathological. Although the nystagmus is typically described by its more easily observable fast (jerk) phase, the salient clinical and pathological feature is the presence of a slow phase in one or both directions. Clinical descriptions of nystagmus are usually based on the direc- tion of the fast phase and are termed horizontal, vertical, or rotary, or any combination of these (Fig. 9-1). The nystagmus may be conjugate or dysconjugate, indicating whether the eyes move 289 290 handbook of pediatric neuro-ophthalmology TABLE 9-1. Nystagmus Types as Identified by History, Physical Examination, and Ocular Motility Recordings. 1. Acquired 2. Arthrokinetic 3. Associated 4. Audiokinetic 5. Bartel’s 6. Bruns’ 7. Centripetal 8. Cervical 9. Circular/elliptic/oblique 10. Congenital/infantile syndrome 11. Convergence 12. Dissociated 13. Downbeat 14. Drug-induced 15. Epileptic 16. Flash-induced 17. Gaze-evoked 18. Horizontal 19. Induced (provoked) 20. Intermittent vertical 21. Jerk 22. Latent/manifest latent 23. Lateral medullary 24. Lid 25. Miner’s (occupational) 26. “Muscle-paretic” 27. Optokinetic 28. Optokinetic after (induced) 29. Pendular 30. Periodic/aperiodic alternating 31. Physiological (endpoint, fatigue) 32. Pursuit (after induced) 33. Rebound 34. Reflex (Baer’s) 35. See-saw 36. Somatosensory induced 37. Spasmus nutans 38. Spontaneous (“voluntary”) 39. Torsional 40. Uniocular 41. Upbeat 42. Vertical 43. Vestibular (central, peripheral, geotropic, ageotropic, galvanic, head shaking, positional, caloric, rotational) chapter 9: nystagmus and ocular oscillations 291 FIGURE 9-1. Diagram of nystagmus in nine positions of gaze. Arrow- heads indicate direction of jerk: fast phase if on one end, pendular nys- tagmus if on both ends, and increasing frequency with more arrowheads. Additional lines indicate increased nystagmus amplitude. The curved lines indicate torsional nystagmus. in synchrony. The nystagmus may be predominantly pendular or jerky, the former referring to equal velocity of the to-and-fro movement of the eyes, and the latter referring to the eyes moving faster in one direction and slower in the other. Involuntary ocular oscillations containing only fast phases are referred to as “sac- cadic oscillations and intrusions” and not nystagmus (Table 9- 2). It is well documented that differentiating true nystagmus from saccadic oscillations and intrusions is sometimes TABLE 9-2. Saccadic Intrusions and Oscillations Identified by History, Physical Examination, and Ocular Motility Recordings. 1. Bobbing/dipping 2. Double saccadic pulses 3. Dynamic overshoot 4. Dysmetria 5. Flutter 6. Flutter dysmetria 7. Macrosaccadic oscillations 8. Myoclonus 9. Opsoclonus 10. Psychogenic flutter (voluntary nystagmus) 11. Saccadic lateropulsion 12. Saccadic pulses/pulse trains (“abduction,” ataxic, intrusions) 13. Square wave jerks/oscillations 14. Superior oblique myokymia 292 handbook of pediatric neuro-ophthalmology impossible clinically. Recent advances in eye movement record- ing technology have increased its application in infants and chil- dren who have disturbances of the ocular motor system.1,4 INCIDENCE In 1991, Stang retrospectively reviewed the records of Group Health Inc. (White Bear Lake, Minnesota, U.S.A.) and in their pediatric population of 70,000 found a prevalence of clinical “nystagmus” of 1 in 2850.66 Other estimates of its incidence range from 1 in 350 to 1 in 6550.11,28,38,41,55 Up to 50% of the infantile strabismic population have some associated nystag- mus.9,20,39,42,44,45,54,60 If these cases are included in the prevalence estimates, then up to 0.5% of the population would be consid- ered to have some form of nystagmus. ETIOLOGY The theoretical neuronal mechanisms of nystagmus continue to evolve and a single unifying explanation is still lacking. However, three major supranuclear inputs to the oculomotor system are clearly important in stabilizing eye movements; their dysfunction may lead to nystagmus. These inputs are the pursuit system, the vestibular system, and the neural integrator. The pursuit system, previously thought to have only a dynamic function, provides a major input for fixation stability (e.g., pursuit at “0 velocity” is stable fixation).37,57 The outputs of the right and left vestibular apparati are neural discharges, each of which tends to drive the eyes contralaterally. Normally the right and the left outputs are equal and cancel each other. Head rotation and unilateral vestibular damage alter this balance. For example, right vestibular damage causes the eyes to drift to the right side. A corrective saccade is then made toward the left. A distinguishing feature of vestibular nystagmus is that the slow phase of the nystagmus toward the affected side is of constant velocity as recorded by electronystagmography. It is important to realize that most forms of acquired nystagmus are caused by disease of the vestibular system (centrally or periph- erally). Eye movement recordings show various combinations of slow phases including uniplanar or multiplanar, simple pendu- lar, linear, and decelerating.52 chapter 9: nystagmus and ocular oscillations 293 The neutral integrator is a theoretical neuronal system that changes the resting firing rate to the extraocular muscles to (1) overcome the viscoelastic forces of the orbit and (2) maintain a position of eccentric gaze. The exact location of the neural inte- grator is unknown, but much of its function resides in the nucleus prepositus hypoglossi (NPH), located just caudal to the abducens nucleus.53,57 Two types of dysfunction of the neural integrator are postulated: integrator leak and high-gain instability. With inte- grator leak, the firing rate of the extraocular muscles is inadequate to overcome the viscoelastic forces of the orbit and maintain the desired eccentric position of gaze; this results in a slow drift of the eyes toward the primary position of gaze and a corrective saccade back toward the desired eccentric position. The slow phase of the nystagmus as recorded by ocular motility recordings is linear (i.e., of constant velocity) or decelerating. Clinically, one observes gaze-evoked or gaze-paretic nystagmus. High-gain instability is a term borrowed from engineering to try to explain the pathophysiology of infantile (congenital) nys- tagmus. In infantile nystagmus, the slow-phase velocity increases as the eyes move from the desired position of gaze; this is referred to as an accelerating slow phase. The ocular motor system (the “output”) is improperly calibrated with the afferent visual system (the “input”). Gain is the ratio of output to input, and in the devel- oping visual system, gain is properly calibrated in the first few weeks to months of life.24,46,73 Faulty calibration of the gain may be a contributing factor in the etiology of infantile nystagmus. The pathophysiology of latent nystagmus and manifest/latent nystagmus (LN/MLN) (referred to by some as fusion maldevel- opment nystagmus syndrome) is different from and less well understood than infantile nystagmus. Because it commonly is associated with the infantile strabismus syndrome, its cause may be related to the documented persistence of nasotemporal motion processing asymmetry that is also characteristic of the syndrome. CLINICAL FEATURES OF NYSTAGMUS IN INFANCY AND CHILDHOOD Neonatal and Early Infantile Nystagmus Distinguishing acquired from the benign neonatal/infantile forms of nystagmus is important because of the implication for under- lying neurological disease in acquired nystagmus (Table 9-3). A 294 TABLE 9-3. Types of Early-Onset Nystagmus. Characteristic INS LN/MLN SN Onset first few months of life Yes Yes Yes May be only visual system Yes Yes Yes deficit (“motor” only) neuro-ophthalmology handbook ofpediatric May be associated with sensory Yes; e.g., albinism, Yes; e.g., strabismic Yes, occasional chiasmal, system disease achromatopsia, aniridia, optic amblyopia hypothalamic, optic pathway nerve hypoplasia gliomas and retinal dystrophy Oscillation characteristics Bilateral, conjugate, symmetrical, Bilateral, conjugate, symmetrical, Low amplitude, high frequency, uniplanar, jerk/pendular uniplanar, jerk (can be worse in dysconjugate, asymmetrical, amblyopic eye) pendular, multiplanar Oscillopsia No No No Worsens with increased fixation Yes No No effort and anxiety Improves with convergence, Yes Yes ??? fatigue, sleep Anomalous head posture Yes, ϳ20% by 1 year of age, Yes, ϳ5%–10%, due to an Yes, usually multiplanar (e.g., a tilt due to “gaze-null”